1. Field of the Invention
This invention relates to coating methods, and more particularly to a coating method which is employed for the manufacture of coated products which are elongated and relatively wide, such as magnetic recording materials.
2. Background of the Invention
In a conventional method of manufacturing magnetic recording materials or the like, coating solutions for forming a magnetic layer, a protective layer and a back layer are continuously and uniformly applied to a belt-shaped support which is elongated and relatively wide. Specifically, the web has a thickness of several micrometers to several hundreds of micrometers, a width of several hundreds of millimeters to several thousands of millimeters, and a length of several hundreds of meters to several thousands of meters. The web is made, for instance, of polyethylene terephthalate. After these coated layers are dried, the web with the layers is wound on a winding core. The web thus wound is longitudinally cut into a number of belt-shaped pieces. Each of the belt-shaped pieces thus formed is further cut to the length of the desired product and then wound on a spool or the like.
A variety of methods such as a roll coating method, gravure coating method, extrusion coating method and slide bead coating method have been employed for coating the web with the coating solution. In these coating methods, in order to form on the web a film which is uniform in thickness in the widthwise direction after drying, the coating operation is carefully carried out with the coating apparatus set with high accuracy.
Recently, in order to mass-produce magnetic recording materials, it has been required in the art to coat a web with a coating solution at high speed in such a manner that the thickness of the film formed on the web is uniform both in the direction of the width and in the direction of the length. In the case of coating a web with a coating solution at high speed, in the roll coating method or gravure coating method the speed of rotation of the roll is considerably high, so that the coating solution is splashed from the roll, thus smudging the web and the peripheral equipment. Also air may be taken into the solution pool, thus forming bubbles or stripes in the film formed on the web. In order to eliminate these difficulties, when it is required to coat a web with a coating solution at high speed, the extrusion type coating method has been employed. Various types of extrusion coating methods have been described (Japanese Patent Application Publication No. 7306/1979, and Japanese Patent Application (OPI) No. 104666/1983 and U.S. Pat. No. 4,681,052).
In the extrusion type coating method, the coating head is pushed against the web in such a manner that a clearance corresponding to the thickness of a film to be formed on the web is provided between the coating head and the web. That is, a solution pool is formed between the coating head and the web. Being formed between the coating head and the web, the solution pool is pressurized. The pressure of the solution pool acts on the web, while the web pushes back the solution pool, so that the coating operation is stably carried out.
In the above-described coating method, sometimes the film formed on the web is not uniform in thickness in the widthwise direction. This difficulty is especially significant when the web is under greater tension on one side than on the other side with respect to a longitudinal line down the center of the web.
In this case, the web pushes the coating head more at the one side than at the other side. As a result, the thickness of the formed film is smaller on the one side where the coating head is pushed more strongly than on the other side. The non-uniformity of the film thickness is sometimes as much as several percent (%) to several tens of percent of the desired thickness, thus providing a serious problem for manufacture of belt-shaped magnetic recording materials or the like.